A new telescope owner pointing at a distant building often concludes something is broken, because the image is upside down or flipped left to right. Nothing is broken. The orientation is a deliberate consequence of leaving optics out of the light path.
Why the image is inverted
A simple telescope forms a real image that is inverted, and an eyepiece magnifies it as it finds it. Correcting the orientation needs extra glass, either an erecting prism or a relay lens, and every extra surface scatters a little light and softens the image slightly. For astronomy there is no up in space, so the correction is left out.
What a diagonal does to it
A star diagonal reflects the light through ninety degrees, which puts the image the right way up and flips it left to right. So a refractor with a diagonal gives a mirror image and a newtonian gives an upside down one, and both are normal. This is why star charts are sometimes printed mirrored and why planetarium software offers a flip option.
When it actually matters
Only when matching what you see to a printed chart, which is a real nuisance for the first month and then stops being one. For terrestrial use it matters a great deal, which is why spotting scopes and binoculars include the erecting optics that astronomical telescopes leave out.
If you want it corrected
An erecting prism, sometimes called an amici prism, gives a fully correct image at a small cost in sharpness and light. It is worth having if the instrument doubles as a daytime spotting scope, and most astronomers do not fit one, because the cost is real and the benefit at night is nil.
Questions people ask about telescope
Why is my telescope image upside down?
Because the correcting optics that would flip it are deliberately left out, since they cost light and sharpness and there is no up in the night sky.
Is a mirrored image a fault?
No. A star diagonal flips the image left to right, which is normal for a refractor or catadioptric used with one.
Can I get a right way up image?
Yes, with an erecting prism, at a small cost in light and sharpness. It is worth it if you also use the telescope in daylight.